1. Field of the Invention
The present invention relates to a radio communication method and a base station for performing radio communications in a cell/sector configuration according to the code division multiple access (CDMA) system using a spectral spread processing.
Especially, the present invention relates to a radio communication method and a base station for transmitting and receiving directional beams using a plurality of antennas.
2. Description of the Related Art
Conventionally, a cellular system for covering the whole service area by disposing a plurality of base stations in the form of a cell is used in a radio communication system.
Recently, a cell/sector configuration is used in the cellular system. The cell/sector configuration separates one cell into a plurality of sectors and disposes a base station antenna in each sector, so as to form the service area.
Generally, as shown in FIGS. 1A and 1B, a three-sector configuration and a six-sector configuration are used in the recent cellular system.
As shown in FIG. 2, all downlink signals DL#1 and DL#2 are spread using scrambling codes and channelization codes over a wide frequency band. A different scrambling code is assigned to each sector, and a different channelization code is assigned to each channel in the sector.
It is possible to suppress an inter-sector interference using the scrambling codes, and to suppress an inter-channel interference in the sector using the channelization codes.
However, the number of channelization codes is limited. Therefore, there is a problem in that the channelization codes can become exhausted when communication traffic increases in the sector.
To solve the problem, the method for using a secondary scrambling code in addition to a primary scrambling code which has already been used in the sector is adopted in the conventional cellular system.
In other words, a different scrambling code is assigned to each channel which is spread by the same channelization code in one sector, so that each channel is identified. However, channels within the sector that are spread by different scrambling codes impose interference to one another.
A directional beam transmission and reception technique is known as a technique for suppressing interference power from other mobile stations (users) in the conventional radio communication system.
In the conventional radio communication system, the base station adopting the directional beam transmission and reception technique transmits the downlink signals to the mobile stations via a plurality of antennas, and receives the uplink signals from the mobile stations via the plurality of antennas.
To be more specific, the base station receives the uplink signals from each mobile station, and synthesizes the uplink antenna weights, so as to form directional beams in the uplink. Then, the base station synthesizes the downlink antenna weights from the uplink antenna weights by compensating for the amplitude and phase variations caused by the RF circuitries, so as to form directional beams in the downlink for each mobile station. The base station receives and transmits the signals using these directional beams. As a result, the interference power from other mobile stations is suppressed.
Conventionally, the directional beam transmission and reception techniques fall into two broad categories: adaptive antenna array systems and multi-beam systems.
The adaptive antenna array system can follow movements of mobile stations by using a different directional beam for each mobile station and by updating the antenna weights continuously, even though the mobile stations move.
For example, a coherent adaptive antenna array (CAAD) reception system using pilot symbols is proposed in the conventional DS-CDMA system.
On the other hand, the multi-beam system prepares fixed directional beam patterns and predetermined antenna weights generating the fixed directional beam patterns. The multi-beam system measures reception power of each directional beam. The multi-beam system performs transmission and reception processing using the directional beam pattern whose reception power has the maximum value.
Moreover, the adaptive antenna array system for performing the transmit diversity in the downlink directions is known.
Channels in the downlink directions fall into two broad categories: dedicated channels and common channels. The dedicated channel transmits signals inherent in each user of the mobile station, and the common channel transmits control signals common to all users.
FIGS. 3A and 3B show examples of transmission beam patterns PT#1 to PT#3 of the dedicated channel and the common channel which are transmitted by the base station adopting the directional beam transmission according to the adaptive antenna array technique.
As shown in FIG. 3A, the base station narrows the transmission beams PT#1 and PT#2 including the dedicated channel of each mobile station MS#1 or MS#2 using the directional beam transmission, so that the inter-user interference power can be suppressed.
On the other hand, as shown in FIG. 3B, the base station transmits the common channel using the non-directional beam PT#3, so that all mobile stations MS#1 and MS#2 can receive the common channel in the sector.
As described above, a unique primary scrambling code and a unique set of secondary scrambling codes are assigned to each sector, and a unique channelization code is assigned to each channel in the sector, in the conventional radio communication system.
The conventional radio communication system makes effective use of the channelization codes by discriminating between a combination of a predetermined channelization code and a predetermined primary scrambling code and a combination of the predetermined channelization code and a predetermined secondary scrambling code, when the channelization code is exhausted in the sector.
As shown in FIG. 4, when a propagation distance between the mobile station MS and the base station BS#1 managing the sector S#1 is different from a propagation distance between the mobile station MS and the base station BS#2 managing the sector S#2, and when the scrambling code used for the channels CH#1 and CH#2 in the sector S#1 is different from the scrambling code used for the channel CH#3 in the sector S#2, the inter-channel interference caused by the channel CH#3 onto the channels CH#1 and CH#2 can be suppressed.
As shown in FIG. 4, however, when the primary scrambling code and the secondary scrambling code are used in the sector S#1, the inter-channel interference caused by the channel CH#1 using the primary scrambling code onto the channel CH#2 using the secondary scrambling code, and vice versa, can remain at a significant level.
Therefore, there is a problem in that the conventional radio communication system cannot make effective use of the channelization codes in one sector, so that a communication capacity of the conventional radio communication system is limited, even though the secondary scrambling code is used in the sector.
In the environment adapting the conventional directional beam transmission, the interference between mobile stations can be suppressed because of the directivity of the transmission beams. However, the problem in that the interference cannot be suppressed sufficiently when the primary scrambling code and the secondary scrambling code are used in the same sector still remains.
Referring to FIGS. 5A and 5b, the above problem will be described in details.
As shown in FIG. 5A, without the use of directional beams, it is impossible to distinctively assign different scrambling codes to each mobile station MS#1 and MS#2 in accordance with a location area of each mobile station MS#1 and MS#2 in one sector, because the transmission beam PT#1 and PT#2 of each channel reaches all mobile stations MS#1 and MS#2 in the sector.
In this case, the channel using the primary scrambling code suffers the interference from all channels using the secondary scrambling codes in the same sector, and the channel using the secondary scrambling code suffers the interference from all channels using the primary scrambling codes in the same sector.
Therefore, the interference between channels using the primary scrambling codes and the secondary scrambling codes increases, so that the conventional radio communication system cannot make effective use of the channelization codes.
On the other hand, as shown in FIG. 5B, the base station transmits the narrowed transmission beams PT#1 to PT#3 including each channel to the mobile stations MS#1 to MS#3, when the directional beam transmission and reception is adopted. Therefore, each channel does not suffer the interference from all channels using the different scrambling codes in the same sector.
However, when there a plurality of mobile stations MS#2 and MS#3 using different scrambling codes in the same direction from the base station, the overlapped transmission beams PT#2 and PT#3 cause the interference.
In other words, the mobile station MS#1 does not suffer the interference, but the mobile stations MS#2 and MS#3 suffer the interference from each other in FIG. 5B.